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Looking Critically at Harvest Weed Seed Control: What Benefits Can be Realized in PNW Dryland Cropping Systems?

Posted by Doug Finkelnburg, University of Idaho | April 7, 2022

Harvest weed seed control has been a popular topic at grower’s meetings this winter and a few early adopters in the PNW are experimenting with this new (to us) technology. While chaff lining, tram lining, narrow-windrow burning, and integrated impact mills are the most referred to under the term “post-harvest weed seed control”, impact mill units have attracted the most buzz at the events I have attended. Regional weed scientists published an excellent Extension Bulletin titled, Harvest Weed Seed Control: Application for PNW (Pacific Northwest) Wheat Production Systems on the practices and given a concise overview in the blog post by Judit Barroso, OSU, titled, “A Potential Tool to Control Herbicide Resistant Weeds During Harvest?” as well. Since there seems to be an abundance of interest, I want to discuss some of the issues surrounding what this technology can and can’t be expected to do for us.

First of all, and key to understanding the potential effectiveness of these technologies, weed seeds need to get into the combine in order to be chaff lined, tram lined, or impact milled. Producers in the southern US using impact mills have had great success controlling herbicide-resistant Palmer Amaranth, but it retains 98% of its seed at the time of harvest. In contrast, our problematic grassy weeds, Italian rye, downy brome, and wild oat tend to lose some significant percentage of seeds to shatter before harvest is begun. The extent of seed drop depends on a number of factors such as weed maturity at time of harvest and localized weather events (think hot, dry August winds or late summer thunderstorms). This has led to skepticism of the effectiveness of harvest weed seed control at making substantial gains in managing problematic grassy weeds considering how a few weed escapes can proliferate, especially if the escaped weeds are herbicide resistant. I agree we should not expect harvest weed seed control alone to solve our grassy weed problems, but there are potential benefits that should be considered before leaving this tool in the box.

Consideration #1:  Volunteer Crop Reduction?

These systems should greatly reduce unintentional re-seeding of fields with the fine fraction of seed harvested, reducing the total area of nutrient robbing and green-bridge disease hosting volunteers that need to be managed.

Consideration #2:  Lower Quality Weed Seeds in the Bank?

We don’t know yet what impact, over time, these practices will have on the soil seed bank. It is possible that the seeds retained on our grassy weeds at time of harvest are more robust and that eliminating them will provide advantages over time by leaving less thrifty weed seeds in the seed bank. This is an area where more research is very much needed on a species-by-species basis.

Consideration #3: Cumulative Benefits?

When used in combination with crop rotation, an herbicide program utilizing multiple-sites of action and other technologies like smart spraying, can we realize benefits over time that allow us to reduce input costs or boost yields? We know there are increased capital and operation costs with adding equipment and practices. We don’t know what the agronomic and economic benefits are over time. Again, this is an area where multi-disciplinary research is needed.

Will harvest weed seed control solve all our weed problems? No, but that doesn’t mean it should be ignored out of hand as a potentially useful tool. As with any weed control activity, be it cultural, mechanical or chemical, the practice is only as good as the extent to which it is integrated into a well thought out integrated weed management program supported by targeted, relevant, and timely research.

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